Improve the Squelch in Your Monitor Receiver

June 11, 1998.

NOTICE: The first version of this article appeared in the
RCMA Newsletter, December 1988. It may not be reproduced
in whole or in part on CDROMS, in bulletin boards,
networks, or publications which charge for service without
permission of the author. It is posted twice monthly on
the USENET groups rec.radio.scanner, alt.radio.scanner,
sci.electronics.repair, and rec.radio.info. It is also
available electronically from the rec.radio.scanner ftp
archive on the official USENET FAQ library
ftp://rtfm.mit.edu/pub/usenet-by-group/rec.radio.scanner.

Introduction

The squelch control in some FM monitor receivers is
difficult to use because of too much hysteresis. The
American Heritage Dictionary of the English Language
defines hysteresis as the:

"Failure of a property changed by an external agent
to return to its original value when the cause of the
change is removed."

It's like having play in a car's steering wheel, or
backlash in a gear set. Too much hysteresis in a squelch
circuit forces one to keep the squelch at a tighter
setting, missing weaker signals when scanning or searching.
The squelch hysteresis problem was first addressed in a
1984 article about the Radio Shack PRO-24 crystal
controlled scanner.

Why do engineers design these circuits with any hysteresis?
Basically, hysteresis prevents weak signals from "popping"
the squelch open and closed rapidly, which would be very
annoying to the listener. A squelch circuit with hysteresis
requires a stronger signal to open the squelch than it does
to keep it open.

Models cursed with too much hysteresis include the
Uniden/Bearcat 800XLT and 760XLT, Radio Shack PRO-2003,
PRO-2004, and others. Don't despair, the fix is simple for
most radios. I've successfully reduced the squelch
hysteresis by replacing a single resistor.

Hysteresis is usually implemented by providing feedback
from the squelch gate input to output, through a resistor.
The higher the resistor value, the less feedback there is,
and less hysteresis. My modification involves replacing the
"stock" resistor with a higher value resistor, as
identified in the table below.

This technique does not apply to some of the newer Uniden
models, because the squelch hysteresis is partially
determined by firmware inside the processor IC.

This article cannot provide step-by-step instructions for
each radio. I always encourage radio hobbyists to purchase
the service manuals for their equipment. The most useful
sections of the manual are the schematics and printed
circuit diagrams. Good manuals will also describe the
theory of operation and alignment procedures, and contain a
complete parts list.

If your radio has too much hysteresis, and is not listed in
the table, study the service manual. Motorola MC3357,
MC3359, and MC3361 integrated circuits, which contain IF
and squelch circuitry, are popular in scanners. In these
receivers, look for the hysteresis resistor to be connected
to these ICs.

Notes

The author cannot furnish modification information
for other radios.

Don't attempt this, or any other internal
modification, unless you are skilled in electronic
servicing.

Make sure your receiver is disconnected from any
power source before opening the cabinet.

If space permits, you can replace the stock resistor
with a higher value variable resistor ("trimpot") and
adjust to suit.